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Symmetry for Control of a Few, Trapped Ultracold AtomsNathan HarshmanProfessor of Physics and Director of NASA DC Space Grant ConsortiumAmerican University
regular polytopes
2
point groups, space groups, kaleidoscopes, matrix groups, Lie groups, symmetric spaces, dynamical catastrophes, random matrices,
Hamiltonian symmetry groups, topological insulators/superconductors…
My Favorite Kind of Mathematical Physics
Top 10 Symmetry Groups forEngineered Quantum Few‐Body Systems
1. Time translations2. Involutions3. Rotations and orthogonal groups4. Symmetric group5. Euclidean group
6. Galilean group7. Coxeter groups8. Homotopy groups9. Unitary groups10. Conformal groups
1. Time translationsOne observable to rule them all: the Hamiltonian
Continuous, abelian group• Real line with addition
• Unitary representation on a Hilbert space
• Derivative is sometimes useful
,tT
† 1U t U t U t 0 0U t U U U t U t
U t U t U t U t U t t
0
( )t
dH i U tdt
†( ) ( ) ( ), 0U t HU t H U t H
Kinematic Symmetry
Goal: find maximal* kinematic symmetry group of Hamiltonian
( ) ( ) KU g H HU g g K
Projective irreducible
representations
Energy eigenspaces
Invariant operators
Conservedquantities
2. InvolutionsStupid baby group for stupid babies.
Also: really important.
Discrete, finite group
• Group multiplication table
• Realizations on space:• reflection, rotation or inversions• represented as orthogonal matrices
• Realizations on Hilbert space or Fock space:• C, P, and T• represented as unitary or antiunitary operators
2 2 1 2 1 (1)Z C D S A O e aa e
Representations of One‐dimensional Parity
• Representation on space
• Induced representation on Hilbert space
• Projection into orthogonal sectors
x x x
2 ( )L ˆ ( ) ( ) ( ) ( )U x x x
ˆ ˆ( )U
12
ˆˆ ˆ( )P 12
ˆˆ ˆ( )P
2 1 ˆˆ ˆ( ) ( )U U †ˆ ˆ( ) ( )U U
ˆ ˆ 0P P
Parity as a Kinematic Symmetry
• Hamiltonian provides decomposition of Hilbert space into energy sectors
• Commutes with Hamiltonian
• Spectral decomposition
ˆ ( ) ( )E Ex x ˆ ˆˆ ˆH H
EE
E EE E
Why *NOT* Symmetry?
FRAGILEthe elephant and the flea
T Dauphinee, F Marsiglio, AJP (2015) 11
3. Rotations andOrthogonal Groups
Three‐parameter compact Lie group• Representation on space
• Lie group• One parameter subgroup• Euler angles
• Induced representation on Hilbert space
(3)R SO
ˆ ˆ( ) exp( )R i L n n( , , ) exp( )exp( )exp( )z y zR i L i L i L
1ˆ ( ) ( ) ( ) ( )U R R x x x
3R x x 3
T TR R RR I det( ) 1R
2 3( )L 1 † 1ˆ ˆ ˆ( ) ( ) ( )U R U R U R
2 1 2 1ˆ ˆ ˆ( ) ( ) ( )U R R U R U R
Irreducible representations • Hilbert space decomposed into sectors
• Rotations leave irrep invariant
• Orbital angular momentum is a good quantum number
( ) ( ) ( , )E m E mR r Y x
( )
'
ˆ ( ) ( , ) ( , ) ( )m m m mm
U R Y Y D R
ˆ ˆ ˆ ˆ( ) ( )U R H HU R
2 1
( )
'( ) ( )m m
mU R m m D R
(3) (2)SO SO
15
Projective Representations
• Wigner 1: Corresponding to a symmetry transformation of physical states is a linear and unitary (or antiunitary) transformation on vectors
• Wigner 2: If there is a group of symmetries, group representation can be projective, i.e. like
• Wigner 3: Projective reps of a simply connected group without central charges are equivalent to (single-valued, true) reps.
• Wigner 4: Projective reps of a group are equivalent to the true reps of the universal covering group of that group.
)(),()()( 212121 RRTRRRTRT
Weyl, Bargmann may have helped.
2 2 2T T
16
Universal Covering Group
• UCG of Lie group is a unique, simply connected Lie group with a homomorphism and isomorphic Lie algebras.
• UCG of SO(n) sometimes called Spin(n)
G G
)3(SO)2(SU is UCG of
G~ G
2SO(3) SU(2) / Z(locally isomorphic to factor group)
Applications of SO(3)
• Single particle in three dimensions, free or spherical trap• Particle on a sphere• Relative motion of two particles in three dimensions and Galilean invariant interaction, free or harmonic trap, e.g. Hydrogen
• Configuration space of three non‐interacting particles in one dimension, free or harmonic trap
• Relative configuration space of four non‐interacting particles in one dimension, free or harmonic trap
Generalize to other dimensions
Generalizations• O(3)
• Rotations• Reflections and rotoreflections, including parity
• SO(4)• Bound states of hydrogen atom• Two subgroup chains
• O(N)• Hyperspherical harmonics
det 1O det 1O
(4) (3) (2)SO SO SO (4) (3) (3)SO SO SO
Dimensionality
• Separate internal discrete degrees of freedom• Separate center of mass DOF for quadratic traps• Separate relative hyperradial DOF for certain traps, interactions
– Remaining DOFs form a sphere• Remove orientation: shape space
19
D\N 1 2 3 4 5 6 7 8
1 1 2 3 4 5 6 7 8
2 2 4 6 8 10 12 14 16
3 3 6 9 12 15 18 21 24
D\N 1 2 3 4 5 6 7 8
1 0 1 2 3 4 5 6 7
2 0 2 4 6 8 10 12 14
3 0 3 6 9 12 15 18 21
Nm Nm
D\N 1 2 3 4 5 6 7 8
1 0 0 1 2 3 4 5 6
2 0 1 3 5 7 9 11 13
3 0 2 4 8 11 14 17 20
Other schemes to separate DOF: adiabatic, Born‐Opp, s‐waves
D\N 1 2 3 4 5 6 7 8
1 0 0 1 2 3 4 5 6
2 0 0 2 4 6 8 10 12
3 0 0 2 6 9 12 15 18
Hyperspherical Harmonics Subgroup Chain
1(1) : 1, 1O
2(2) : 0,1,2,3,...O
1( ) 1d
2(0) 1, ( 0) 2d d
3(3) : 0,1, 2,3,...O 3 3( ) 2 1d
3(4) : 0,1,2,3,...O 2
4 4( ) 1d
4(5) : 0,1,2,3,...O 5
4
25 4 5 5 5
0
( ) (2 1)( 1) / 6d
(5) (4) (3) (2) (1)O O O O O
4. Symmetric GroupThe master of all finite groups
Symmetric groups
12 (1)(2)p 21 (12)p
123 (1)(2)(3)p 213 (12)p 231 (123)p
1234 ( )p 2134 (12)p 2314 (123)p
Objects Elements ConjugacyClasses
2 2 2
3 6 3
4 24 5
5 120 7
2143 (12)(34)p 2341 (1234)p
One irreducible representation for every conjugacy class
Standard Young Tableaux
• Young diagram:• There are N boxes in rows and columns.• Upper left justified• Each row has the same or fewer number of columns as row above.• Unique correspondence between partition of N and Young diagram
• Standard Young tableau:• Fill boxes with all numbers 1 through N used only once.• Numbers must increase to the right and to the bottom.
• Conjugate partitions :• Conjugate diagrams have rows and columns reversed• Conjugate diagrams have same number of standard Young tableaux
Symmetric Group Irreps
1 21
2[2] 2[1 ]
1 2 33
1 2
2
1 31
2
3
[3] 3[1 ][21]
[4]
2[21 ]2[2 ]
1 2 3 43
1 2 4
4
1
3
3
2
3
1 2
4
2
1 3 4
4
1 2 3
[31]
2
1 3
4
1
3
4
3
1
2
3
4
1
2
3
4[1 ]
22
2
S (1 ) (2)[2] 1 1[1 ] 1 1
33 1 3 2
3
S (1 ) (12) (3)[3] 1 1 1[21] 2 0 1[1 ] 1 1 1
Character tables
Irrep and character facts
• Dimensions of irreps
• Orthogonality of irrep characters
• Useful special case
( ) ( )[ ] ( ) ( ) [ ]i i ii
g g g G
2 [ ]n G
2( )[ ] ( ) [ ]i ii
g g G
4 2 24 6 3 8 6
2
2
4
S (1 ) (1 2) (2 ) (13) (4)[4] 1 1 1 1 1[31] 3 1 1 0 1[2 ] 2 0 2 1 0[21 ] 3 1 1 0 1[1 ] 1 1 1 1 1
A
B
C
D
5 3 2 25 10 15 20 20 30 24
2
2
3
5
S (1 ) (1 2) (12 ) (1 3) (23) (14) (5)[5] 1 1 1 1 1 1 1[41] 4 2 0 1 1 0 1[32] 5 1 1 1 1 1 0[31 ] 6 0 2 0 0 0 1[2 1] 5 1 1 1 1 1 0[21 ] 4 2 0 1 1 0 1[1 ] 1 1 1 1 1 1 1
Five is different
Applications
•Particle permutation symmetry
Normal Exchange Statistics:particle permutations of identical particles given by symmetric group
• Bosons
• Fermions
• Parastatistics (useful for partially distinguishable identical particles)
30
Np S
1 21 2 1 2ˆ ( ) ( , , ) ( , , ) ( , , )
NN p p p NU p x x x x x x x x x
1 21 2
1 2
( , , ) evenˆ ( ) ( , , )( , , ) odd
NN
N
x x x pU p x x x
x x x p
1 2 1 2ˆ ( ) ( , , ) ( ) ( , , )i N ij j N
jU p x x x D p x x x non‐ abelian
abelian
Applications
• Particle permutation symmetry• State permutation symmetry
Double Tableaux Basis
Applications
•Particle permutation symmetry• State permutation symmetry•Non‐interacting particle symmetry
Wreath Products
What is the group of symmetry transformations?
1 3D S3
3 1S D
31D
3Spermutations reflectionspermutations and reflections don’t commute
semidirectproduct
Non-interacting identical quantum systems
35
T St N
Applications
•Particle permutation symmetry• State permutation symmetry•Non‐interacting particles•Ordering permutation symmetry
( )i iV q q ( )i iV q q
2( )i iV q q 10( )i iV q q10 0
( ) 0| |
iii
ii
qqV q
Disconnected Ordering Sectors
37
Unitary limit: Ordering permutation symmetry
3!tT S
38
Ordering Permutation Symmetry
123
213
132
231
321
312
1 2st nd
1 2st nd
1 2st nd
2 3nd rd
2 3nd rd2 3nd rd
Ordering permutations are different from particle
permutations
40
Symmetric Well Asymmetric Well
1‐2 and 2‐3 tunneling rates the same 1‐2 and 2‐3 tunneling rates different
Harshman, FBS (2017)
Near‐Unitary Limit
Four particles
41
Symmetric Well
Asymmetric Well
Square Well
Two parameters for four particles in symmetric trap
42
Applications
•Particle permutation symmetry• State permutation symmetry•Non‐interacting particles•Ordering permutation symmetry•Well permutation symmetry
2 2
2 2
2 2
00
a a A a b a BD b C
a A a b a BD C
a a A a b a BD b C
KT P T W T P
P T W PT O T W T O
2 2
2 2
2 2
00
a A a b BD b C
A a b BD C
a A a b BD b C
KT P T W T P
P T W PT O T W T O
2 2
2 2
2 2
00
a a A a b a b BD b b C
a A a b a b BD b C
a a A a b a b BD b b C
KT P T W T P
P T W PT O T W T O
2
2 2
2 2
00
a ABCD
AC a BD
a A AC a BD
KT P W
P W T P WT O W T P W
2
2 2
2 2
00
a ABCD
AC a BD
a A AC a BD
KT P W
P W T P WT O W T P W
2
2 2
2 2
00
a a ABCD
a AC a a BD
a a A AC a a BD
KT P W
P W T P WT O W T P W
5. Euclidean GroupNot a simple, compact Lie Group
Euclidean group
• Isometries of space
• Matrix representation
• Semidirect product
x x Rx a y x y x ( ), NR O N a T
, , ,R a R a R R a R a ,0 ,0 ,0R R R R , , ,I a I a I a a
( ) ( ) NE N O N T
( ) ( )E N ISO N
1 11 12 1 1
2 21 22 2 2
1 0 0 1 1
x R R a xx R R a x
Applications
• Free space in N dimensions• Two and three dimensions well‐studied for 2000+ years
• Subgroups include point groups and space groups• Classified (but not counted)
• Configuration space• Relative configuration space
• Rotations in space shape!• Relativistic quantum mechanics
• Little group of photon
6. Galilean Group
Galilean Symmetry
• Translations in space
• Translations in time
• Rotations
• Galilean boosts
t t b Ĥ ˆ ˆ( ) exp( )U b ibH
x x a P̂ ˆ ˆ( ) exp( )U i a a P
R x x Ĵ ˆ ˆ( ) exp( )U R i θ J
t x x v ˆ ˆmK Xˆ ˆ( ) exp( )
ˆexp( )
U i
im
v v K
v X
Kinematic symmetries of free‐space Hamiltonian
Dynamic symmetries of free‐space Hamiltonian
Complete Set of Commuting Observables forQuantum One-Body Problem
• For one‐body problem in free space
• For spherical symmetry
• For central potential
ˆˆ{ , }zSX ˆˆ{ , }zSP2
0ˆˆ ˆ ˆ{ , , , }z zH L SL
2 20
ˆ ˆ ˆ ˆ{ , , , }zH JJ L
ˆ ˆ, 0H J2 2ˆ ˆ ˆ ˆ{ , , , }zH JJ L
ˆˆ ˆ ˆ, , 0H H L S2 ˆˆ ˆ ˆ{ , , , }z zH L SL
7. Coxeter GroupsUpon further reflection…
21, , ijr i i j m e
Coxeter groups realized as geometric reflection groups in Laplacians with delta ridge potentials
( )xH c n x s
2 22 1 2 1 2( ) ,I e D
2 2
(2 1)222n
n iH x nL x L
m x
• Same a contact interaction, but different masses with same trapping frequency
• Hard core limit• Solvable and (maximally super) integrable when masses have right size and right order
2 2 2
1 ,
1 1 ( )2 2
N
i i i i ji i ji
H p m x g x xm
Coxeter Model
g
NL Harshman, M. Olshanii, A Dehkharghani, A Volosniev, SG Jackson, NT Zinner, PRX (2017) 56
Natural coordinates Mass‐rationalized Jacobi coordinates
Relative, mass‐rationalized Jacobi coordinates
( )tan j i j kijk
i k
m m m mm m
Unequal Masses
57
12,34
123132
234324
13,2412341324
1243
1342
2134
3124
14231432
23143214
2143
3142
Three particles: quantum billiards on a ring
Four particles: quantum billiards in spherical triangles
( )tan j i j kijk
i k
m m m mm m
, 2ij kl
Five particles: quantum billiards in 3‐spherical tetrahedra 58
3A 3C 3H
Four equal mass
particles
Not possible with four particles
But…
: tetrahedral : octahedral : icosahedral
maximally superintegrable, exactly solvable
59
•Angles
•Reflections
Coxeter Groups and the Method of Images
3 [3,3]A 3 [4,3]C 3 [5,3]H
123 234 12,34[ ,3] , ,3 2q
q
3 223 12 34 23 12 34[ ,3] ( ) ( ) ( ) 1
qq R R R R R R 60
1
2
3
4
3A
3C
3H
mass fractions
rational masses exist for A‐series and C‐series. Not others.
61
62
algebraic solvability, separabilityLiouville integrability, Bethe‐Ansatz integrability, superintegrability, maximal superintegrability,
ergodity, mixing, chaos!
8. Homotopy GroupsThe “aid” groups
Generalized Exchange Statistics:occur when configuration space *not simply connected*
Three reasons this can occur:1. Underlying space has non‐trivial topology (e.g. wells, ring, torus)2. Hard‐core interactions create defects in configuration space3. Indistinguishable particles induce non‐trivial topology on
configuration space
64
• Make space not connected• Order points in space, left or right• No phase relation in disconnected sectors
Defects with co‐dimension onePoint in a line Line in a plane Plane in a space
• Make space not simply‐connected, path space not connected• Order paths in space, winding numbers• No phase relations among inequivalent paths
Defects with co‐dimension twoNot possible
in 1‐DPoint in a plane Line in a space
Defects with co‐dimension threePoint in a space
• Make space not simply‐contained, path space not simply connected• Order surfaces in space, wrapping numbers• No phase relations among inequivalent surfaces
Not possible in 1‐D
Not possible in 2‐D
1 1 1i i i i i is s s s s s
Breaking the Symmetric Group
• Symmetric group– Generators– Relations
68
1 2 1, , , Ns s s
2 1is when 2i j j is s s s j i
NS 1 2(12), (23),s p s p
11 1s s
1 2 1 2 1 2s s s s s s 1 3 3 1s s s s
Three ways to break symmetric group
21 1 1 23, ,n n i i i i j i iS s s s s s s s e
break
1 1 1 23, ,n n i i i j i iB s s s s s s e
21 1 2, ,n n i i j i iT s s s s s e
21 1 1 3, ,n n i i iF s s s s s e
Braid group
Traid groupaka twin group
Fraid group
Traid and fraid are hyperbolic Coxeter groups
11 1b b
11 1t t
1 2 1 2 1 2b b b b b b 1 2 1 2 1 2t t t t t t
braid group traid group
11 1s s
1 2 1 2 1 2s s s s s s
symmetric group
Topology of Hard Core Particle Models
71
2‐body 3‐body 4‐body
1 2 3
2 4 6
3 6 9
1d
2d
3d
co‐dimension of defect caused by local few‐body
hard core interaction
Configuration space is
disconnected: defects like line in plane, planes in space
connected, butnot simply connected:
defects like point in plane,lines in space
everything else,simply connected:
defects like point in space
i j
i j
x x
y y
i j k
i j k
x x x
y y y
Defect co‐dimensiondoes *NOT* depend on N
Bose‐Fermi mapping
Braid anyons
Traid anyons
One more case: fraid anyonsnon‐local 2‐2 interactions
9. Unitary GroupsMatrix, Lie, compact and useful for harmonic oscillators
Harmonic oscillators
• Kinematic symmetry of N‐dimensional isotropic harmonic oscillator
• Lie algebra of quadratic operators
(2 ) (2 , ) ( )O N Sp N U N
2 2 † †1 1
1 1 / 22 2
N N
i i i ii i
H p q a a N
a a
Rotations in phase space
Canonical linear transformations
† †( )ij i j j i i j j iL Q P Q P i a a a a † †( )ij i j i i j jC N N a a a a
† †( )ij i j i j i j j iD Q Q PP i a a a a
Compact Lie Groups from Simple Lie Algebras
• A series
• B series
• C series
• D series
• Exceptional Lie groups
74
1 2 3, , ,A A A
2 3 4, , ,B B B
3 4 5, , ,C C C
4 5 6, , ,D D D
2 4 6 7 8, , , ,G F E E E
SU( 1)r
SO(2 1)r
Sp(2 )r
SO(2 )r
10. Conformal Groups
Scale transformation
• Consider the differential operator
• Stretching operator
xx
n nx x nxx
( ) ( )x
xe f x f e x
Scale dilations in free space QM
• Consider the free space Hamiltonian
• Scale transformation leaves Schrodinger equation invariant
• Dilation operator
0 2 i i ji i jH V r rm
2,r r t t
2V r V r with
1 ( )2 i i i ii
Q r p p r 0 0[ , ]Q H H
Calogero interaction2‐D delta function1‐D delta prime
Can also add harmonic trapping potential
Applications
1. Entanglement in scattering systems2. Contact interactions in one dimension3. Double‐well control dynamics4. Tight‐binding models in one dimension5. Anyons from hard‐core interactions